Description Cassiopea is a genus of scyphozoan jellyfish very commonly found in shallow mangrove swamps, mudflats, and turtle grass flats in Florida and various other similar environments around the world, where it lives usually upside-down on the bottom. Where found, there may be numerous individuals with varying shades of white, blue, green and brown. They have a mild sting since they are primarily photosynthetic, but sensitive individuals may have a stronger reaction. The stinging cells are excreted in a mucus; swimming over these jellies (especially using swim fins) may cause transparent, essentially invisible, sheets of this mucus to be lifted up into the water column, where they are then encountered by unsuspecting swimmers. The stings, appearing in the form of a red rash-like skin irritation, are notorious for being extraordinarily itchy.

The Cassiopea jellyfish (one common species is Cassiopea andromeda) belongs to the Order Rhizostomeae and mostly lives in sandy areas and seagrass beds. The Cassiopea jellyfish is also called "Upside Down Jellyfish", because it lies on its back, so that the bell touches the ground. In this position it resembles a sea anemone. Sometimes this jellyfish is picked up by a crab (Dorippe frascone) and carried on its back. The crab uses the jellyfish to defend itself against possible predators.

Warning Mildly toxic. Contact with the Upside-down Jellyfish causes itching followed by a rash.